1. Field of the Invention
The invention relates to a method for identifying a heartbeat, the method comprising measuring an arterial pressure signal in the form of a multichannel measurement, and carrying out signal-processing operations, such as filtration, if necessary, and performing signal detection and decision-making concerning the identification of a signal as a heartbeat signal.
The invention also relates to an apparatus for identifying a heartbeat, the apparatus comprising a multichannel sensor, preferably a signal-processing means for processing a signal obtained from the multichannel sensor, detection means for detecting a signal, and decision-making means for identifying the signal as a heartbeat signal.
2. Brief Description of the Prior Art
Prior art conventional arrangements for heartbeat measurement performed on an arterial pressure pulse are based on the use of one sensing element, in other words the measurement is carried out via one channel. The problem is, however, that heartbeat measurement, based on the use of one sensing element, from an arterial pressure pulse is only accurate when the subject is at rest and interference is minimal. The movement of a limb, a tendon or a muscle creates significant interference in the signal measured by the sensor, which may result in an unsuccessful measurement. The problem is aggravated by the similarity in frequency bands and waveforms of the interference and the actual signal in an individual measurement channel, which makes it more difficult to distinguish the signal from interference.
Arrangements utilizing multichannel measurement are also known. Multichannel measurement of a pressure pulse provides better capacity than a one-channel method. In a multichannel pressure pulse signal, the interference is mainly common-mode or similar amongst different channels, which enables elimination of interference caused by movement of the subject. A known method is disclosed in U.S. Pat. No. 5,243,992. The method disclosed in this reference utilizes a multichannel tonometer sensor. The signal processing utilizes known methods for decreasing interference signals, such as band-pass filtration and subtraction of an average of signals of all channels from each channel signal. In U.S. Pat No. 5,243,992, the heartbeat detection is based on the selection of the best signal channel. The pulse in the selected channel is accepted as a heartbeat pulse by means of a correlation function. A few preceding pulse amplitudes are used as the selection criteria for the channel. Such selection of one channel leads to uncertain operation during interference situations and when the sensor moves with respect to the artery. Due to the high interference level, the method based on the selection of one channel may lead to the selection of a channel other than the one with the best heartbeat signal. Also, when the heartbeat of a moving person is measured, the movement of the sensor at the wrist with respect to the artery may transfer the heartbeat signal to a different channel, and therefore the signal may be lost for a while until a channel providing a good signal is located again. Also, the use of mathematical operations, such as a correlation function, requires a great deal of calculation capacity, in which case implementation of a signal processor is required, which in turn leads to a large size and great consumption of energy. Other known multichannel measurement methods are disclosed in EP 404,594 and SE 425,290.